There have been several frosts over the past week. Although it was probably not cold enough to cause major damage in most areas, it is possible flower buds may have been affected if you are in a particularly frost prone area or if plants were more advanced (e.g. over solid black ground cloth). In some cultivars, I have many dead buds in Simcoe (Norfolk County), but it is unclear if the damage is from frost or an issue that I had last year. Frost damaged buds will turn brown and shrivel up, but you may have to pull back the upper leaves to see the damage. If this occurred, there may still be flower buds lower in the canopy that are intact. It is also possible the plant will produce new buds that will bloom sporadically throughout the summer. There is not much that can be done about this kind of damage.
Severely frosted plants would have obvious damage to all the new, green growth. This growth would be wilted and turn brown, defoliating the plant. I have not had any reports of this kind of damage. If this occurred to lavandins, they would likely need to be replaced, although it is best to wait a few weeks to see how they respond. For angustifolia, they will grow back from new buds on the woody stems near the base. Mowing or pruning the plants within 10 cm of the ground would help get light down to the centre of the plant and speed recovery.
Now is the type to be scouting for four-lined plant bug nymphs. Early instar (stage) nymphs are mostly bright red and wingless. As the nymphs mature through 4 instars, they gradually grow black wings and become a duller rusty-red colour (Figure 1). They are likely already causing damage to the tips of the new shoots. This damage appears as a spotting and distortion of the newest leaves (Figure 2). The damage will begin to appear on older leaves and stems as the nymphs mature.
Figure 1. Early feeding damage to lavender caused by four-lined plant bug.
Figure 2. Early damage caused by four-lined plant bug feeding appears as spotting and distortion of the newest leaves at the tips of the shoots.
There are no products registered for control of four-lined plant bug on lavender for culinary use. However, it is possible insecticidal soaps applied for aphid control on lavender may suppress the smallest nymphs if you can get good coverage of all surfaces of the plant. If the plants are only to be used as a cut flower and not for culinary use, then there is a much wider range of insect control products, mostly conventional, that can be used. This would be listed on the label as “outdoor ornamentals” and can be used as long as there is not an exclusive list of crops that does not include lavender, or the label specifically precludes use on “cut flowers”. If you had severe damage last year, you may want to consider this option for that area. Contact me if you would like more information on this option.
The other potentially damaging insect that will begin to affect lavender now is the garden fleahopper. These insects can reach very damaging levels, especially over black ground cloth. The plants in my research trial were completely defoliated by garden fleahoppers in August last year but recovered after I applied a product registered for outdoor ornamentals. For more information on garden fleahoppers visit here: https://www.ontario.ca/page/garden-fleahoppers-lavender
If you have not done so already, now is a good time to make the first application of nitrogen (N) to your fields. My research has shown an optimal rate of 80-100 kg/ha N over the season every year. It is probably best to split this amount over two or three applications between now and mid-August. The rate can be adjusted depending on the size of the plants and the method of application. Visit this post for more information on calculating fertilizer rates: https://onspecialtycrops.ca/2020/05/14/fertilizer-calculations-for-lavender/